The iframe page tab is not compatible with the Send dialog.
I followed the example on the 'Send Dialog' page https://developers.facebook.com/docs/reference/dialogs/send/
<html xmlns:fb="https://www.facebook.com/2008/fbml">
<body>
<script src="http://connect.facebook.net/en_US/all.js"></script>
<div id="fb-root"></div>
<script>
// assume we are already logged in
FB.init({appId: '123050457758183', xfbml: true, cookie: true});
FB.ui({
method: 'send',
name: 'People Argue Just to Win',
link: 'http://www.nytimes.com/2011/06/15/arts/people-argue-just-to-win-scholars-assert.html',
});
</script>
</body>
</html>
However, this resulted in an error:
API Error Code: 3
API Error Description: Unknown method
Error Message: Unknown method
When I changed to the 'feed' dialog, everything worked perfectly.